The present invention relates to an electronically controlled fuel metering system.
Many types of aircraft, such as helicopters, typically use an electronically controlled fuel metering system that fails at a fixed metered level of fuel in the event of losing primary power. Additionally, upon losing primary power, it is highly desirable that the subsequent transfer to backup mode be completed in a manner that will not require an additional workload to be imposed on the pilot of the aircraft. A backup mode is an alternative operational mode that is executed when a primary (or a normal operational mode) encounters a partial or total failure. This type of smooth transfer to a backup mode is facilitated by a fuel metering system that does not change or impede fuel flow. This type of unchanging, non-impeded fuel flow is known as a bumpless transfer. A bumpless transfer is also a transient free or smooth transfer that is transparent to a user or system undergoing the bumpless transfer.
According to an aspect of the present invention, a fuel metering device for metering fuel to an engine provides bumpless transfer to backup. This fuel metering device comprises a common cavity for providing fuel flow pressure relief, fuel metering and bypass of fuel flow to a flow of fuel entering the common cavity. A metering valve schedules fuel flow to the engine and the metering valve is controlled in either a primary mode or a backup mode. A pressure relief valve controls and limits the maximum pump discharge pressure of a fuel pump. A bypass valve maintains a constant pressure across the metering valve by redirecting non-metered fuel flow back to a pump stage inlet. An electrical clutch selectively determines whether the metering valve function is accomplished by the primary mode or the backup mode; and the bumpless transfer to the backup mode is accomplished automatically.
According to another aspect of the present invention, a fuel metering method for metering fuel to an engine provides bumpless transfer to backup. The method includes forwarding fuel flow into a common cavity and providing pressure relief, fuel metering and bypassing of fuel flow in a common cavity. Fuel flow is scheduled to the engine and the metering valve is controlled in either a primary mode or a backup mode. The maximum pump discharge pressure of a fuel pump is controlled and limited. A constant pressure is maintained across the metering valve by redirecting non-metered fuel flow back to a pump stage inlet. An electrical clutch is used to selectively determine whether the metering valve function is accomplished by the primary mode or the backup mode; and the bumpless transfer to a backup is accomplished automatically.